A particle moves according to the law of motion $s(t) = t^{3}-6t^{2}+4t$, $t\ge 0$, where $t$ is measured in seconds and $s$ in feet.

a.) Find the velocity at time $t$.
b.) What is the velocity after $3$ seconds?
At $t_1=$ and $t_2=$ with $t_1.
When $0\leq t<$ and $t>$